The present invention relates to a method for separating propane and propylene contained in a gas mixture.
The propylene polymerization reaction for obtaining polypropylene generally uses:                propylene, as monomer,        a catalyst (Ziegler-Natta, metallocene, chromium oxide, etc.),        an agent controlling the length of the polymer chains, such as hydrogen, and        inert substances issuing or not from the reaction (propane, nitrogen, etc.).        
During this polymerization reaction, unreacted propylene is usually recycled to the inlet of the polymerization reactor. This recycling involves a step of cooling or condensation of the effluent from the polymerization reactor, in order to maintain a constant reaction temperature. The propylene is recycled with other components of the reactor effluent such as hydrogen, propane and nitrogen. The propane recycling is supplemented by a make-up of very rich and very pure propylene. Although this make-up is very rich in propylene, inert substances, such as propane and nitrogen, or light compounds, such as hydrogen, accumulate in the recycling effluent, thereby considerably reducing the productivity of the polymerization unit. To decrease the concentration of these inert substances or these light compounds, the recycling loop is occasionally purged of these substances. In other words, a part of the recycling loop is sent to a monomer recovery unit or flared. This purge operation has the drawback of decreasing the profitability of the polymerization process because either an additional unit is dedicated to propylene recovery, or the propylene is lost at the flare. To decrease these losses, it was proposed in patent U.S. Pat. No. B1-6,271,319 to treat the recycled effluent with a membrane permitting the selective permeation of the propylene with respect to the propane. Thus, the membrane serves to obtain 1) a propane-enriched retentate which can thus be removed from the recycle loop and 2) a propylene-enriched permeate which can be recycled to the polymerization reactor. The propylene losses are therefore limited in comparison with the prior art and the productivity of the polymerization process is improved. The drawback of this solution is that the membranes described in U.S. Pat. No. B1-6,271,319 consist of materials selected from polyimides, polyphenylene oxides and perfluoropolymers, which are slightly permeable to propylene. In consequence, for a given installed membrane surface area, the propylene recovery rate remains low.